This invention relates to a system for handling treatment materials such as those used in removing multivalent heavy metals and metal cyanide ions from metal plating waste effluents. More particularly, this invention relates to a system for stabilization and removal of exhausted vermiculite particles from a cation exchange column and for drying and, possibly, expanding the stabilized particles to produce a useful byproduct.
Metal plating is used to improve surface properties of metallic and nonmetallic products by coating a relatively thin, adherent layer of metal onto an object. Electroplating is the most common and important of the various metal plating processes. Metals commonly used in electroplating processes include nickel, copper, chromium, zinc, gold, silver, cadmium, and tin.
There are two basic types of metal plating baths used in electroplating processes. They are the simple salt (or "acid" bath) and the complex ion bath, with cyanide being the most commonly used complex ion. For example, copper can be plated from either an acid or an alkaline bath. If an acid bath is used, the following simplified explanation is typical of the plating process. Two electrodes are immersed in a copper sulfate solution and connected to a direct current electrical source. When current is applied, copper ions in solution migrate toward the negative electrode (cathode) which can be the article to be plated. The positive electrode (anode) is of copper and is the source of new copper ions in solution to replace those which are plated onto the article.
When an alkaline plating bath is used, cyanide is the anion in solution and forms a complex with the heavy metal ion to be plated. Commercial alkaline copper and zinc metal plating baths usually contain cyanide as the complexing ion; cadmium plating baths almost always use cyanide. Typical cyanide concentration in such baths may range from 15,000 to 100,000 mg/l.
Ion exchange has been utilized to concentrate both cyanide-heavy metal complexes and heavy metal ions in plating waste effluents to facilitate their later removal or recovery. An important advantage of ion exchange treatment is the savings of water due to recirculation of treated water. However, past methods using ion exchange have also suffered shortcomings including the presence of impurities in the waste which are destructive to ion exchange resins, the presence of interfering ions, a limited loading capacity of ion exchange columns, and relatively high operating costs. Even after regeneration, the metal ion precipitated from the spent regenerant solution constitutes a sludge that is difficult to dispose of in an acceptable manner.
U.S. Pat. No. 4,100,065 to Etzel represents an improved cation exchange process for removing heavy metal ions from metal plating waste effluents. That process involves the use of exfoliated vermiculite particles as the cation exchange material. A number of ways to treat the exhausted exfoliated vermiculite particles are mentioned by Etzel. Included is the possibility of regenerating the particles for reuse as ion-exchange resins. Another possibility listed is to further expand the ion-depleted vermiculite and, then, use it as a lightweight insulation, packaging material or filler material.
In Etzel et al U.S. Pat. No. 4,210,530 there is disclosed an improvement on the process of U.S. Pat. No. 4,100,065. That improvement is to use an unexpanded vermiculite as the cation exchange material. Again the disclosed methods for handling of the exhaused vermiculite particles include regeneration and expansion of the removed particles.
However, in practice, these handling methods are not as easily accomplished as might at first appear. Regeneration for the purpose of reuse is not economically feasible. The amount of chemicals used, the costs involved, and the disposal problems created by the used regenerating solution, all make it unfeasible at the present time to regenerate the exhausted vermiculite particles.
Accordingly, disposal of the exhausted vermiculite is the only viable approach. Even then, the exhausted vermiculite particles must be removed from the column. The problem encountered is that the wet vermiculite particles cannot be easily removed from the column in condition for ready disposal. The wet particles are difficult to dewater, dry and/or transport. More importantly, removal of the wet vermiculite particles results in carrying some of the heavy metal contaminants out of the column. These heavy metal ions are ones which have not been fixed to the vermiculite particles by the ion exchange process, of if theoretically attached, they are easily leached from the particles under ordinary disposal conditions, although, ordinary water washes are not sufficient to rid the spent vermiculite of these contaminants. This means that the exhausted vermiculite particles cannot be simply removed and sent to a land fill because of the presence of possible toxic leachants.
Accordingly, the need exists for an efficient and effective means for removal and treatment of exhausted vermiculite particles from an ion exchange column which will result in production of a useful byproduct or at least one which may be disposed of safely.